A fluorine-containing ion exchange membrane is excellent in e.g., heat resistance and chemical resistance. Therefore, the fluorine-containing ion exchange membrane has been used not only as a cation exchange membrane for alkali chloride electrolysis for producing chlorine and an alkali but also a diaphragm for generating ozone, a fuel cell, wide variety of diaphragms for electrolysis such as water electrolysis and hydrochloric acid electrolysis. Of them, the membrane for use in alkali chloride electrolysis is demanded to, e.g., increase current efficiency in view of productivity, reduce electrolysis voltage in view of economic efficiency and reduce the concentration of sodium chloride in caustic soda in view of quality of a product.
Of these demands, in order to increase current efficiency, an ion exchange membrane formed of at least two layers, i.e., a carboxylic acid layer using a carboxylic acid group having high anion elimination property as an ion-exchange group and a sulfonic acid layer using a low resistant sulfonic acid group as an ion-exchange group, is generally used. Since these ion exchange membranes are brought into direct contact with chlorine and caustic soda of from 80 to 90° C. during an electrolysis operation, a fluorine-based polymer having extremely high chemical resistance is used as a material for the ion exchange membrane. However, the ion exchange membrane formed of such a fluorine-based polymer alone does not have sufficient mechanical strength. Therefore, the membrane is reinforced, for example, by embedding a woven fabric contained of polytetrafluoroethylene (PTFE) in the membrane, as a reinforcing core material.
For example, Patent Document 1 proposes a fluorine-based cation exchange membrane for electrolysis composed of a first layer, which is formed of a fluorine-based polymer film having a cation-exchange group and reinforced with the woven fabric, and a second layer, which is formed of a fluorine based polymer having a carboxylic acid group and positioned on the cathode side, in which ≧½ of the thickness of a porous base material is projected from the first layer toward the anode side, the projecting part of the porous base material is covered with a coating layer of the fluorine-based polymer having the cation-exchange group so as to integrate into the first layer and to form the convexo-concaves along with the surface shape of the porous base material on the anode side surface.    Patent Document 1: Japanese Patent Application Laid-Open No. 4-308096